The development and function of tissues depend on interactions between non-parenchymal and parenchymal cells to modulate differentiation, proliferation, and migration. Specifically, parenchymal-non-parenchymal interactions are important in physiology, pathophysiology, cancer, development, and in attempts to replace tissue function through ‘tissue engineering’. While the functional importance of such cell-cell interactions is well established in many systems, the underlying molecular mechanisms often remain elusive. Parenchymal cells interact with extracellular matrix materials, non-parenchymal cells and soluble signals to properly differentiate and maintain parenchymal cell functions. A great deal of study has been dedicated to the identification of extracellular matrix materials that play a role in parenchymal tissue development and maintenance. Such studies have elucidated the role of certain factors in parenchymal cell maintenance and development, however, there remains many unanswered questions as to the role of soluble factors and membrane bound interactions with non-parenchymal cells in the development of tissue, tissue maintenance and tissue growth.
The ‘feeder layer’ effect is widely used in stem cell culture and the culture of many tissue types (skin, liver, pancreas, muscle and the like). This is often used as a generic support of differentiated (or self renewing) cells much in the way serum is a generic additive for cell culture. Identification of the factors (soluble or insoluble) would be important for laboratory and therapeutic applications.
The liver is the heaviest gland of the body, weighing about 1.4 kg (about 3 lb) in the average adult, and is the second largest organ after the skin. The lobes of the liver consist of many functional units called lobules. Each lobule consists of specialized epithelial cells, called hepatocytes (i.e., parenchymal cells), arranged in irregular, branching, interconnected plates around a central vein, the sinusoids, through which blood passes.
The liver's main function is to control the level of particular substances in the blood. For instance, the liver plays a major role in carbohydrate metabolism by removing glucose from the blood, under the influence of the hormone insulin, and storing it as glycogen. When the level of glucose in the blood falls, the hormone glucagon causes the liver to break down glycogen and release glucose into the blood. The liver also plays an important role in protein metabolism, primarily through deamination of amino acids, as well as the conversion of the resulting toxic ammonia into urea, which can be excreted by the kidneys. The liver also detoxifies many drugs and hormones. In addition, the liver participates in lipid metabolism by storing triglycerides, breaking down fatty acids, and synthesizing lipoproteins. The liver also secretes bile, which helps in the digestion of fats, cholesterol, phospholipids, and lipoproteins; and stores vitamins (A, B12, D, E, and K) and minerals (iron and copper). Furthermore, the Kupffer's cells of the liver phagocytize worn-out red and white blood cells as well as some bacteria. Bilirubin, a breakdown product of heme, is excreted by the liver into the bile ducts where it passes into the intestinal tract.
There are many different causes of liver disorders. Hepatitis, an inflammation of the liver, is commonly caused by alcoholism or other toxic ingestion, or infection by viruses or other parasites. Cirrhosis of the liver is marked by the destruction of parenchymal cells and their replacement with connective tissue. Hepatitis resulting from infection by hepatitis C virus, for example, often develops into cirrhosis. Hepatitis B virus infection, on the other hand, is strongly believed to lead in many cases to liver cancer (hepatoma). Hepatoma can also be caused by the activation of endogenous oncogenes, through exposure to carcinogens, for example.
Severe forms of these disorders may result in chronic or acute hepatic failure. Fulminant hepatic failure (FHP) is associated with massive necrosis of hepatocytes and concomitant sudden severe impairment of hepatic metabolism. Partial or total liver replacement is needed in case of transient or permanent failure of vital liver functions.
The liver has a remarkable-capacity to regenerate. In the rat, for example, a 70% hepatectomized liver will regenerate its original mass in about seven days. Nonetheless, because the liver carries out so many important biochemical functions, severe liver damage or loss of the liver is rapidly fatal. Some efforts have been made, therefore, to identify the molecular factors involved in the liver regeneration process.
Most previous studies have focused on events occurring in the first few (e.g., 1-6 hours) hours after surgery. Identifying genes that regulate hepatocyte fate and function (i.e., proliferation and differentiation) has been very difficult. One study examined hepatocyte proliferation in an in vivo regeneration model. In this regard, Hagiya et al., 1994, Proc. Natl. Acad. Sci. USA 9:8142-8146, cloned ALR (augmenter of liver regeneration) from rat; Hsu et al., 1992, Mol. Cell Biol. 12:4654-4665, identified a gene encoding a novel leucine-zipper containing protein termed liver regeneration factor-1 (LRF-1); and Mohn et al., 1991, Mol. Cell Biol. 11:381-390, identified 41 novel immediate-early partial DNA sequences. These genes were isolated by examining expression during early time periods following partial hepatectomy (e.g., 1-6 hr). During this early stage of regeneration, the expression of acute phase inflammatory proteins is substantially high, thereby yielding a “background” of induced expression of genes which are not very useful because their expression is not specific to liver regeneration. This approach is limited for several reasons: (i) it is not causal, there is no way to filter the genes to establish a relationship to hepatocyte fate (i.e., pro- or anti-regenerative), (ii) changes in gene expression are combined for all cell types in the liver (hepatocytes and others, which comprise 33% of the liver), and (iii) there is large variability between animals. Overcoming this lack of specificity may be a determining factor in obtaining urgently needed tools for early diagnosis of liver disorders, as well as improvements in therapy that take advantage of the liver's unusual regenerative capacity.